Study On Preparation And Property Of V-Doped And Nd/V Co-Doped Bismuth Titanate Ceramics

It was reported in Nature and Science in 1999 and 2002 respectively that La doped bismuth titanate (Bi_3.25La_0.75Ti₃O₁₂) films showed well-saturated hysteresis loops and excellent ability of fatigue-free, revealing the essential applications of the film in ferroelectric random access memories (FRAMs). Since then, more and more attention has been paid to this kind of material. It is recognized that bismuth titanate (Bi₄Ti₃O₁₂) is the simplest compound and a well-known member among the bismuth layer-structured ferroelectrics, which has been mostly investigated up to now. It has been pointed out that Bi₄Ti₃O₁₂ (BIT) exhibits the good properties with a low dielectric constant, a high Curie temperature (675℃), and big self-polarization that could cause it to be a suitable material widely used in the fields of capacitor, high-temperature piezoelectric sensor and optic-electric devices. In the view of environment protection, lead-free property of BIT also make it to be as an essential substitute of PZT, which is now used in commercial functional ceramics field. However, the disadvantages of BIT, such as high dielectric loss, low remnant polarization (Pr) and high coercive field (Ec) as well as its layer-structured characteristic limit its commercial application. On the other hand, the high conductivity is also its shortcoming in the application of sensor. In order to overcome the property limitation of Bi₄Ti₃O₁₂, doping with additive in the composition of Bi₄Ti₃O₁₂ is considered as to be an available approach. In fact that La doped bismuth titanate film with the excellent properties mentioned above just is a perfect example to improve its property by addition of doping in the composition.In the present thesis work, the effects of V-doping(BTV) and Nd/V(BNTV) co-doping on the preperation and property of bismuth titanate ceramics were studied, The investigations that have been carried out includes the sintering behavior, phase identification, microstructure observation and property characterization of doped bismuth titanate ceramics.Firstly, the study of V-doped BBi₄Ti₃O₁₂, (formula Bi₄Ti_3-xV_xO_12+x/2 (x≤0.08) (BTV) ceramics that were prepared by solid reaction method was performed. The results indicated that single BIT phase was obtained for the powder calcined at 800℃for 1 h for the composition with x≤0.08. Besides, the V-doping does not show the change on lattice parameters of BIT phase by XRD. The measurement of Raman spectra proved that the vibration of lattice of BIT was influenced by coping of vanadium. On the other hand, the sintering ability of the samples were greatly improved by addition of a little amount of V, since the relative density of the sample can reach as high as 98% at 900°C for lh. The observation of microstructure of the samples indicated that the size of grains becomes smaller with uniformly distribution by increase of V amount in the composition. For BIT ceramics (x=0.08) the second phase, Bi_2.45Ti_4.05V_0.09O₁₂, was found by SEM. In addition, V-doping led to an improvement in the decrease in conductivity of the samples, while the high Curie temperature could be almost unchanged. When x=0.02 and 0.04, the piezoelectric constant (d₃₃) of the material being 20pC/N was obtained.Using the similar technique, Nd/V co-doped BIT ceramics(BNTV) (formula Bi_4-xNd_xTi_2.98.V_0.02O_12.01) were investigated, in which the doping amount of V was fixed as 0.02, together with amount of Nd these are x=0.16, 0.26, 0.36 and 0.56 in four compositions. The addition of Nd in the composition resulted in the decrease in Curie temperature of the samples that depended on increasing Nd concentration. However, the loss tangent of the materials were much lower (1.8% at 400℃/100KHz for BNTV0256) than that of BIT samples (77%). Besides, BNTV ceramics exhibited a saturated ferroelectric hysteresis loops with 2Pr of 27-31μC/cm and Ec of 41-57.5Kv/cm.Secondly, the preparation of doped BIT powders was tried to get a new way to improve the property of the ceramics by using this kind of powder as starting materials.Vanadium doped Bi₄Ti₃O₁₂ powder was prepared by a co-precipitation method and its phase evolution process, microstructure and sintering behavior were investigated. The results indicated that Bi₄Ti₃O₁₂ crystalline phase with grain size of less than 100nm could be obtained by calcination of the precursor at 550℃, which is 250℃lower than that of traditional solid reaction. The powder possessed a good sintering activity and bulk density of sintered sample can reach 96% of theoretical value when sintering at 900℃for 1h. In comparison to the material prepared by solid reaction, the sample starting from co-precipitation powder also has lower dielectric loss.Thirdly, the study of preparation of Nd/V co-doped BIT particles by molten salt method was carried out and the particles were considered to be used as seed in the further investigation of textured co-doped BIT ceramics. BNTV particles were successfully synthesized in different chloride salts (NaCl and KCl with different molecular ratios) fluxes. XRD and EDS results revealed that small radius Na⁺ entered the grain and made the lattice parameters fall down evidently. The second phase Na_0.5Bi_4.5Ti₄O₁₅ was found in the particles formed in NaCl flux. The morphology of grains formed in KC1 flux is near square (rectangle), while it is close to roundness with average size 5-8μm formed in NaCl flux. The grain size of BNTV increased with the increasing of amount of NaCl in the fluxes, but the thickness decreased. Raising temperature in KCl flux can make the grains bigger with the shape of roundness.